The metal ion in the active site of the enzyme carbonic anhydrase is zinc Zn . The specific coordination chemistry of zinc in the active site involves a tetrahedral coordination geometry. The zinc ion is coordinated to three histidine amino acid residues His94, His96, and His119 in human carbonic anhydrase II and a water molecule or hydroxide ion HO/OH as the fourth ligand.The coordination of the zinc ion in the active site plays a crucial role in the enzyme's ability to facilitate the reversible hydration of carbon dioxide CO . The mechanism of carbonic anhydrase can be described in two main steps:1. Activation of the water molecule: The zinc ion polarizes the coordinated water molecule, making it more nucleophilic. This leads to the deprotonation of the water molecule by a nearby amino acid residue usually a glutamate or aspartate residue , resulting in the formation of a zinc-bound hydroxide ion Zn-OH .2. Nucleophilic attack on CO: The zinc-bound hydroxide ion acts as a nucleophile and attacks the electrophilic carbon atom of CO. This results in the formation of a bicarbonate ion HCO , which is then released from the active site. The enzyme returns to its original state with a water molecule coordinated to the zinc ion, ready for another catalytic cycle.The tetrahedral coordination geometry of the zinc ion in the active site of carbonic anhydrase is essential for the enzyme's function. It allows for the proper orientation and activation of the water molecule, as well as the stabilization of the transition state during the nucleophilic attack on CO. This coordination chemistry enables carbonic anhydrase to efficiently catalyze the reversible hydration of CO, playing a vital role in processes such as respiration, acid-base balance, and CO transport in living organisms.